Steady Partner, Steady Seasons

Overview

Our Moon acts like training wheels for the Earth, allowing for a stable cycle of seasons. In part A, children ages 11 to 13 model how Earth's tilt creates the seasons. They use their bodies to review the Earth's daily motions before investigating the reason for Earth's seasons in this kinesthetic exploration. The motion of the Earth about its axis (rotation) and in orbit around the Sun (revolution) is related to the appearance of the sky over the course of the day and year. In part B, children model that if its tilt was not stabilized by Moon, Earth's axis would slowly wobble between straight up (0° tilt) to nearly on its side (80° tilt). The resulting seasonal extremes would be unfavorable for life. Allow 1 hour for this activity.

Note that this activity is appropriate for older children who are able to explore the geometry of Sun-Earth-Moon relationships in three dimensions. Many children under 10 are not able to fully conceptualize the Earth's spherical nature and their relationship to it, and so they are unable to create an accurate mental model. This activity models complicated motions that are more advanced than those performed in Spin! Day and Night.

What's the Point?

Seasons are caused by the tilt of Earth on its axis and not by Earth's distance from the Sun, which is roughly consistent throughout the year.

The Sun's path across the sky changes slowly over the seasons. When the Sun is high in the sky in the Northern Hemisphere during summer, temperatures are warmer because the Earth's surface receives more direct sunlight and days are longer. When the Sun is low in the sky in the Northern Hemisphere during winter, the less-direct sunlight and shorter days lead to colder temperatures.

Earth's tilt is stabilized by the Moon's gravitational pull on Earth.

Without the Moon's gravitational pull, Earth would be a very different place. The climate would vary wildly as Earth's tilt changed from 0° to 80° over thousands of years. The changes in seasonal extremes would be very challenging for life, if it existed at all.

Models — such as the children are creating here with their bodies — can be tools for understanding the natural world.

Materials

For the group:

An indoor or outdoor space large enough for the children to form a circle with arms outstretched to their sides

Preparation

Review the background information.

Provide an indoor or outdoor space large enough for the children to form a circle around the pumpkin. They must be able to stand with their arms outstretched and not touching. The experience will be most effective with at least eight children participating.

Select a direction to represent Polaris. The children should be able to lean about 23.5° from upright to point their heads at it.

If possible, hang the constellation signs in order around the room in a counterclockwise direction.

Print, and if desired, laminate the Earth's Continents. Punch a hole in each of the upper ("northern") corners. Suspend the two pages from the strings to create a wearable sign. Set the pumpkin "Sun" on an elevated surface, such as a table, so that it is about the height of a child's navel.

Facilitator's Note: While the Moon and Earth do not physically touch, they are linked by an invisible, but important, force: gravity. The Moon's gravitational pull on Earth plays an important role in how the Earth moves — they are linked in an eternal "dance."

Activity

Part A: Earth's Tilt Creates the Seasons
1. Explain to the children that they will use their bodies to model how Earth's tilt creates the seasons. Because the Moon and Earth are in motion together as they orbit the Sun, this model will be similar to a dance. Invite the children to form a circle around the "Sun" pumpkin. Explain that each child’s upper body represents planet Earth. Ask the children to make sure they stand with enough room between each other to spread their arms out at shoulder height. Remind the children of the scale model they created in Earth's Bright Neighbor.

What's a model?

We use models to help us represent objects and systems so that we can study and understand them more easily. Scientists use computers to create models to study the complex interactions between the Sun, Earth, and Moon.

Where is your North Pole? Top of the head.

Where is your South Pole? At the bottom of the spine (the tailbone).

Where is the equator? It is an imaginary line drawn where the chest meets the belly and all the way around the belly.

What hemisphere is above the equator? Northern.

What hemisphere is below the equator? Southern.

Where is North America? On our chests.

Distribute a sticker to each child and have everyone place them in the center of their chests (right over the breast bone/sternum). Explain that, for this model, the sticker represents "home."

South America? Lower left belly.

What is on the other side of Earth from North America? China and Asia.

Where is Australia? Lower right back —" Down Under."

”Home” is in which hemisphere? Northern.

What time of day is it at “home”? Midday.

How far would a blueberry-sized Earth really be from a pumpkin-sized Sun? About 491 feet or 150 meters (about three blocks).

2. Have the children consider the Earth's motion relative to the pumpkin "Sun." They should be able to spin counterclockwise to model the cycle of day and night, as they did in the activity Spin! Day and Night.

How does the Earth move? It spins (rotates) on its axis.

What do we call this time period? A day.

Earth has another cycle of motion that you have experienced only 10, 11, 12, or 13 times. What is it called? A year.

How long does it take for Earth to go once around the Sun? 365 days or 1 year.

Invite the children to model this motion by walking counterclockwise in a circle around the pumpkin "Sun."

How many seasons are in a year? In what order do we experience them? The four seasons repeatedly cycle through spring, summer, fall, and winter.

What changes do we observe in nature as seasons change? The children may associate many changes with the seasons. In the summer, days are long and the Sun is high in the sky around lunchtime. Temperatures are at their warmest. Animals and plants are growing and active. In winter, days are short and the Sun remains low to the horizon, even at midday. Temperatures are at their coldest. Animals may hibernate and plants tend to be dormant or less active. Spring and fall are transitions between these extremes and may be associated with growth and harvest, respectively.

What do the children think causes the seasons?

Assure the children that their model accurately represents Earth’s orbit around the Sun as nearly circular. Earth is about the same distance from the Sun at any point during the year.

Facilitator's Note: The children may have many misconceptions related to seasons:

It is not true that the Sun rises exactly in the east and sets exactly in the west every day.

It is not true that we experience seasons because of the Earth's changing distance from the Sun (closer in the summer, farther in the winter); in fact, the Earth is closer to the Sun during the Northern Hemisphere winter than it is during the Northern Hemisphere summer!

It is not true that the seasons change because Earth's tilt changes.

It is not true that the seasons change because the length of the day changes.

It is not true that the amount of daylight increases each day of summer.

These and other education misconceptions are listed at http://amasci.com/miscon/opphys.html by Operation Physics, an elementary/middle school physics education outreach project of the American Institute of Physics, 1825 Connecticut Ave. NW, Suite 213 Washington, DC 20009 (202) 232-6688.

The Earth's orbit is not quite a perfect circle: It is slightly closer to the Sun during the Northern Hemisphere's winter. The seasons are caused solely by Earth's tilt rather than this small change in distance. In contrast, seasons on Mars are influenced by both the planet’s tilt and changes in its distance from the Sun (due to its elliptical orbit).

3. Invite the children to model the seasons by adding a key component to their "dances": Earth's tilt!

We have been modeling Earth's spin by standing straight upright. Is this how the Earth is oriented relative to the Sun-Earth plane? No, it is tilted.

How much is Earth tilted? 23.5°.

Show the children the tilt of a mounted Earth globe and point out the direction you have chosen to represent Polaris.Invite them to be acrobatic as they model Earth's tilt with their bodies! They will need to keep their "north pole" pointed toward Polaris, and their "south pole" pointed away from Polaris. The angle will dictate that the children bend their bodies various amounts backwards, to the side, and forwards, depending on their positions in the circle. Have the children rest periodically as they explore the seasons kinesthetically.

Explain that it is a coincidence that Earth's axis is pointing toward Polaris, or the North Star. Polaris has long been used for navigation because it is a convenient nighttime marker for north. The children should bend at the waist to point their heads in the direction of Polaris. The angle is about halfway between 90° and 45°. Help the children experience with their bodies the correct tilt by pointing out adjustments as necessary.

Facilitator's Note: Polaris is very far away from Earth — it takes light, the fastest thing in the universe, 500 years to get there — so the children should all be roughly parallel to each other, rather than converging on a nearby point.

Why does the Earth have a tilt? The children may have many ideas. Some may recall that other planets are also tilted relative to the Sun-Earth plane; for instance, Uranus is tipped on its side with a tilt of 98°!

Add that scientists use computer models and lots of math to calculate how planets changed since they first formed 4.6 billion years ago. Collisions like the one that formed the Moon were common in the early days of the solar system. Repeated large collisions probably added up to give the planets each their own unique tilt. Who knows what the Earth’s tilt was like before the Moon-forming impact, but it certainly would have contributed to the angle we see today!

4. Challenge the children to (carefully) model Earth's spin while maintaining the proper tilt. Ensure that they keep their heads pointed in the direction of Polaris as they bend at the waist and spin, counterclockwise, in place. If desired, have the children move slowly through one day: pause at sunset (Sun low along their right hands), midnight (spines facing the Sun and now bending in an opposite direction), and sunrise (Sun low along their left hands). Have them return to midday at "home" and their original bends.

Facilitator's Note: Throughout this activity, gently suggest corrections to the children's body positions. Ensure that the children keep their heads pointed toward the point you have chosen to represent Polaris whenever they are modeling Earth's motions. They may need reminders to keep their heads straight in line with their bodies. They may also need encouragement to maintain a bend that is not too far forward at the waist or too upright, but about 23.5°. Putting Earth's motions together is a challenge — but one they will remember!

5. Optional: Challenge the children to model both the day and year while maintaining the proper tilt. Invite the more adapt "dancers" to step toward the "Sun" and form a smaller, inner circle. Have them try spinning through a few days while also moving in orbit around the Sun (also counterclockwise) and pointing their heads toward Polaris. If others wish to attempt the full motions, have them step forward in turns to the inner circle. After they have had adequate time to explore these motions, have everyone return to midday at "home."

6. Have the children relate Earth's tilt to our seasons by evaluating their orientations relative to the Sun. Use the Seasons Signs to label the transitions between seasons in the circle as you discuss them.

What are you tilted toward? Polaris.

Consider how your body is bent relative to the Sun. Who has his/her "Northern Hemisphere" (upper body) bent most directly toward the Sun? Identify the child with the greatest forward bend. Who has his/her "Northern Hemisphere" (upper body) leaning most away from the Sun? Identify the child on the opposite side of the circle with the greatest back bend.

Facilitator’s Note: Use careful language when discussing Earth's orientation with this kinesthetic activity. The Earth is tilted toward Polaris, but its hemispheres lean toward or away from the Sun. Earth's tilt remains constant over the course of a year. Earth’s movement around the Sun carries each hemisphere, in turn, to a different orientation relative to the Sun.

Orient the globe in the same position as the children as you discuss how their bodies model the seasons.

Hold the globe near the child bending toward the Sun. What season does North America experience when Earth arrives at this position in its orbit? Summer. What name do we give the first day of summer? Summer solstice. What season does South America experience at this time? Winter.

Place the "Solstice: June 21" sign on the floor near the child.

Hold the globe near the child leaning away from the Sun. What season does North America experience when Earth arrives at this position in its orbit? Winter. What name do we give the first day of summer? Winter solstice. What season does South America experience at this time? Summer.

Place the "Solstice: December 21" sign on the floor near the child.

What is different about the way the children are standing? One is bending toward the Sun, the other away from it.

What is the same about the way the children are standing? Their heads point toward Polaris.

Do they each have one hemisphere closer to the Sun than the other? Yes.

Remind the children of a shortfall of this model: it is not to scale. Show the grape or blueberry to the children and ask them to recall that it is the proper size of Earth relative to the pumpkin "Sun," placed 491 feet or 150 meters (about three blocks) apart. Tip the grape or blueberry slightly to show Earth’s tilt.

Does the slight change in the distance between the part of the blueberry leaning toward the Sun and the other half make a difference when the Sun is so far away? No.

Proceed counterclockwise around the circle and hold the globe near the child leaning sideways toward Polaris. What season does North America experience when Earth arrives at this position in its orbit, halfway between winter and summer? Spring. What name do we give the first day of spring? Spring equinox. What season does South America experience at this time, halfway between their summer and winter? Fall.

Place the "Equinox: March 21" sign on the floor near the child.

Proceed counterclockwise around the circle and hold the globe near the child leaning sideways toward Polaris. What season does North America experience when Earth arrives at this position in its orbit, halfway between summer and winter? Fall. What name do we give the first day of spring? Fall equinox. What season does South America experience at this time, halfway between their winter and summer? Spring.

Place the “Equinox: September 21” sign on the floor near the child.

7. Investigate how the Sun appears lower in the sky — and for shorter days — during winter and higher in the sky — and for longer days — during summer.

Ask the children standing near the "Solstice: December 21" sign to look at the "Sun" while doing the appropriate backbend. Does the "Sun" appear high or low in their "skies" from "home" in the Northern Hemisphere, i.e. do they have to look slightly up or down to see the pumpkin? The “Sun” is low — they have to look down.

Ask the children standing near the "Solstice: June 21" sign to look at the "Sun" while doing the appropriate forward-bend. Does the "Sun" appear high or low in their "skies" from Mt. Nose in the Northern Hemisphere, i.e. do they have to look slightly up or down to see the pumpkin? The "Sun" is high — they have to look up.

Invite the children to walk, counterclockwise, around the circle so that all the children have the opportunity to stand near the June and December solstice signs and experience the "Sun" high and low in their "skies."

When is the Sun highest in the sky? While leaning toward the Sun.

What season is it? Summer.

In what season is the Sun lowest in the sky? Winter.

Ask the children to think of their own experiences. During which season are days longer, allowing them to play outdoors longer and perhaps go to bed later? Summer.

Facilitator's Note: Because of the tilt of the Earth's axis, those living in the northern hemisphere in winter see the Sun rise in the southeast and then set after a short day in the southwest. In the summer, the Sun rises in the northeast and sets after a long day in the northwest; during the day, the Sun moves significantly higher in the sky than in winter and the day is significantly longer. On the first day spring and fall, the Sun rises due east and sets due west, with a day that is about 12 hours long.

The higher the Sun is in the sky, the more intense the sunlight is. During summer, the Sun is at its highest and the temperatures are warmer than during winter.

8. Ask the children to use their own experiences to connect the concept of less sunlight to colder temperatures, and vice versa. Have them recall the time of day when they experience the coldest temperatures — mornings and evenings — when the Sun is low in the sky. Contrast the cooler times with high noon.

During what time of day is the sunlight most direct, in the morning or around noon? In the morning.

Using the dance circle, can the children identify the time of year when the sunlight is most direct, i.e., when is "home" pointed most directly toward the "Sun"? Summer.

Are the days long or short in summer? Long.

Explain that the direct sunlight and longer days of summer give us the warmer temperatures.

What causes the colder temperatures of winter in the Northern Hemisphere? There are fewer daylight hours and the Sun is lower in the sky. The sunlight is less direct.

Part B: What If There Was No Moon?9. Imagine what Earth's seasons might be like if there was no Moon to keep its spin fairly steady. Describe how Earth wobbles over a very long time — 26,000 years. In the past, Earth's North Pole pointed not roughly at Polaris, but at a different star, and sometimes at just the blackness of space! The gravitational tug of the Moon acts to stabilize this wobble. Without the Moon acting like training wheels on the spinning Earth, Earth's tilt could range from 0° to 80°! Invite the children to bend fully at the waste and predict what seasons would be like with that orientation.

What would summer be like in the Northern Hemisphere if Earth was tilted by 80°? Would the Sun ever set? What would that make temperatures like? The Sun would always be high above the horizon, making for very hot summers!

Would the Sun ever rise in the winter? No.

Invite the children to stand upright.

Is one hemisphere more directly facing the Sun than the other? No.

Is the height of the Sun in the sky on Mt. Nose different than it would be on a mountain in the Southern Hemisphere? No.

Would there be seasons on an Earth with no tilt? No.

Add that Jurassic dinosaurs thrived in a relatively hot climate and Ice Age mammoths were adapted to a cold climate, but these different animals lived millions of years apart. Invite the children to imagine a moon-less Earth, which would probably experience similar extremes over a shorter period of thousands, rather than millions, of years.

What do the children think would have happened to the Jurassic dinosaurs if the Earth’s tilt had slowly changed to 80°? Would the heat-loving dinosaurs have been able to survive winters where the Earth leaned almost completely away from the Sun?

Facilitator’s Note: The Moon's gravitation pull on Earth stabilizes long-term changes in how severe or mild the seasons are. The Sun and other planets — especially the giant planets, Jupiter, Saturn, Uranus, and Neptune — also gravitationally influence Earth. The giant planets exert enough of a gravitational tug that they could change Earth's tilt over time. Without the Moon's nearby influence, a planetary tug-of-war between the giant planets would slowly tip Earth from between 0° to 80°. The changes would occur slowly, so that Earth would still experience a consistent climate pattern on a yearly basis. However, life that evolved when Earth had a tilt of 23.5° would be unable to survive the shift in climate thousands of years later due to an 80° tilt, or vice-versa.

Conclusion

Regroup the children and discuss Earth’s seasons — and how different it would be without the Moon’s gravitational pull acting as training wheels! If desired, reinforce the children's conceptions of Earth's seasons by viewing the animation of the spinning, tilted Earth. Note that the movie has been "sped up" to show one day. Invite the children to note how the tilt of the Earth exposes Antarctica to the sunlight.

What time of year do the children think this animation shows? Summer in the Southern Hemisphere (winter in the Northern Hemisphere).

Add that the images were taken in December (1990) by the Galileo spacecraft on its way to Jupiter.

Ask one of the more adept "dancers" to model a December day by spinning counterclockwise and leaning away from the pumpkin "Sun" by 23.5°.

Scientists use computer models to help them understand the relationship between the Sun, Earth and Moon — just like the children used their own bodies! They want to know more about how Earth's current tilt and rate of spin are related to the Moon-forming impact. They use mathematics to describe how the motions, mass, speeds, and directions of the Sun, Earth, and Moon interact in space, and the computer combines all of the factors together to show the overall effect. Mathematics and computers are tools for looking at the past, into the future, and considering what might have been!

Provide the children with theirWithout the Moon... 3comic panels and art supplies. Allow them time to color in the panel about how different it would be on Earth with erratic climate changes over time. Instruct them to add the panel as the next page in the Marvel Moon comic book by clipping the book together at the upper left corner.

Invite the children to return for the next activity, Dance of the Moon and Oceans to discover how the Moon creates tides — and what our world would be like without the Moon's gravitational tug on our oceans.